442 ORIGIN OF THE APPALACHIAN COAL STRATA, 



a regularly progressive expansion of the calcareous rocks. In the 

 following tabular statement, which refers chiefly to the southern 

 counties of Pennsylvania, this gradation is rendered strikingly 

 obvious. 



TABLE FIRST, 



Skotiitig the gradual Increase in the aggregate thichtess of the Limestoiie, as we cross 

 the Southern Coal-fields of Pennsylvania, westward. 



Broad Top basin ; half way across the Appalachian chain, .... nojie. 



Potomac basin ; near the main escarpment of the Alleg-heny mountain, about 10 feet. 

 Eastern basin of Somerset county, west of the escarpment, and about twelve 



miles west of the Potomac basin, 12 feet. 



Ligonier basin, twenty miles west of the last, 30 feet. 



Second western basin, on the Youghiogheny river, fifteen miles west of the 



last, ■ • about 40 feet. 



Great basin of the Monongahela and Ohio rivers, at Brownsville, probably. CO feet. 



Same basin at Wheeling, about 200 feet. 



The above aggregates admit the more accurately of compar- 

 ison, since most of them refer to the same portion of the formation, 

 that, namely, which is included between the gi-eat conglomerate, 

 and the top of the main limestone, above the Pittsburg coal-seam. 



TABLE SECOND, 



Sfiowing the Gradation in the thickness of the large Limestone stratum, overlying the 

 Pittsbtirg Coal-seam. 



Cumberland basin ; not more than 

 Eastern Somerset basin, not determined, but 

 Liponier basin, — average about 

 Monongahela and Ohio basin, at Brownsville, 

 Same basin at "Wheeling, .... 



Inthe upper coal group, or that part of the series which com- 

 mences with the Pittsburg seam, the total thickness of pure lime- 

 stone, excluding numerous thin bands, associated ^^^.th some of 

 the layers of shale, is not less than one hundred and fifty feet.* 

 Some of the limestone strata of the coal-measures, possess, as will 

 be seen from the second of these tables, a remarkably wide dis- 



♦ See Report on Geological Survey of Virginia, for I&IO 



